1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and components and, more particularly, to an etalon with temperature-compensated cavity length and a post-assembly fine-tuning adjustment.
2. Description of the Related Art
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity. Wavelength division multiplexing (WDM) is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In WDM optical systems, multiple information-carrying channels may be transmitted along the same optical fiber, each channel comprising an optical signal of a specific restricted wavelength range. Thus, WDM allows transmission of data from different sources over the same fiber optic link simultaneously, since each data source is assigned a dedicated channel. The result is an optical communication link with an aggregate bandwidth that increases with the number of wavelengths, or channels, incorporated into the WDM signal. In this way, WDM technology maximizes the use of an available fiber optic infrastructure, so that what would normally require multiple optic links or fibers instead requires only one.
The capacity of a WDM system is a function of the number of channels that can be carried in a single fiber, and is limited by the channel spacing between wavelength channels. Such wavelength channel spacing is generally defined by an international telecommunications union (ITU) grid. For example, one ITU grid used by WDM optical systems has a channel spacing requirement of 100 GHz, where the spacing is referenced in terms of a frequency spacing corresponding to a channel center wavelength spacing of 0.8 nm. Another such ITU grid has a channel spacing requirement of 50 GHz, thereby allowing a single fiber to carry twice as many channels as a fiber operating at a 100 GHz spacing. Because different portions of a WDM optical system may operate with different channel spacings, optical interleavers are used in such systems to combine multiple wavelength channels carried on two fibers at a wider channel spacing, e.g., 100 GHz, into a single optical signal that is carried on a single fiber and having a narrower channel spacing, e.g., 50 GHz. Similarly, optical deinterleavers are used in WDM systems to divide a multi-channel optical signal having a narrow channel spacing into two multi-channel optical signals having wider channel spacing and carried on two separate fibers.
One device commonly used in the art as part of interleavers/deinterleavers is the Fabry-Pérot interferometer, also known as an etalon, which is a structure that introduces a phase difference between the even and odd channels of a multi-channel optical signal to facilitate physical separation of the channels making up the signal into two groups. Interleavers/deinterleavers commonly incorporate one or more etalons to increase the width of passbands and isolation bands of the individual wavelength channels, thereby providing greater ease of alignment of the channels to an ITU grid. As channel spacing becomes increasingly narrow, e.g., 50 GHz and smaller, subtle factors significantly degrade etalon performance, such as small manufacturing variations of the etalons and drift caused by thermal expansion or contraction of etalon components during normal operation. For example, the center wavelength for a 25 GHz interleaver is preferably stabilized to within ±1 GHz of the desired ITU-defined value, but ambient temperature change is known to cause etalon cavity length to vary sufficiently to result in a 2-3 GHz center wavelength shift of the etalon. Similarly, minor but unavoidable manufacturing flaws can produce unwanted offset between the actual center wavelength of an etalon and the desired center wavelength. While an etalon is commonly tuned to an ITU grid during assembly by precisely positioning components of the etalon, such as a tuning plate, an epoxy baking process typically takes place after such a tuning procedure, and even extremely small relative movement between etalon components during such a process can result in the center wavelength of the etalon being permanently fixed with a significant offset from the desired center wavelength value for the etalon.
Accordingly, there is a need in the art for an etalon with temperature-compensated cavity length and/or a post-assembly fine-tuning adjustment.